The aim is to study the levels of underlying oxidant stress in critically ill neonates with pulmonary hypertension. It will also be determined whether inhaled nitric oxide (NO) treatment of neonates with PPHN is associated with increased levels of potentially toxic NO-derived oxidants, decreased platelet aggregation, or the release of vasoconstrictive endothelium derived hormones. The relationship of potential NO toxicity (thiol and nitrotyrosin function, impaired clot retraction, and stimulation of endothelial-1 release) and measures of short term outcome (number of ventilator days, length of O2 treatment) will be measured.